We have determined the ordering properties of water adsorbed at room temperature on the rock salt (100) surface under four different conditions: ultrahigh vacuum, dry nitrogen atmosphere, 45% and 75% relative humidity. Details of the atomic structure are determined for both sides of the solid-liquid interface. The top most layer of NaCl shows a small relaxation that changes from an expansion to a contraction with increasing humidity. Under all measured conditions water monolayers with different ordering properties are present at the interface. Surprisingly, we find that the amount of ordering in the first layer is increasing with increasing thickness of the water film. At a solid-liquid interface, the ordering appears to be correlated with the solubility. (C) 2004 American Institute of Physics.